Row address code selection based on locations of substandard memory cells

ABSTRACT

A memory device identifies memory blocks that contain substandard memory cells. The memory device then determines row address codes to apply to the memory blocks during refresh operations. The row address codes determine which memory blocks of the memory block are refreshed together. The row address codes are designed to ensure that memory blocks having substandard memory cells, which must be refreshed more frequently than other cells, are refreshed together, while memory blocks without substandard memory cells are refreshed together.

CROSS-REFERENCE TO RELATED APPLICATION

This U.S. non-provisional patent application claims priority under 35U.S.C. §119 to Korean Patent Application No. 10-2009-0088368 filed Sep.18, 2009, the subject matter of which is hereby incorporated byreference.

BACKGROUND

Volatile memories such as dynamic random access memory (DRAM) continueto play a major role in modern electronics due to their high integrationdensity and operating speed. These memories are commonly used, forinstance, as the main memories, caches, and data buffers of personalcomputers, workstations, gaming-consoles, and handheld devices, to namebut a few.

One major drawback of volatile memories is that their memory cells losestored data in the absence of applied power. This data loss typicallyoccurs because stored electrical charges tend to diffuse away from theconstituent memory cells, allowing stored data to fade over time. DRAMcells, for instance, store data by placing electrical charge oncapacitors, and the stored information fades as the charge dissipates orleaks away from the capacitors.

To prevent stored information from being lost entirely, volatile memorycells may be periodically refreshed (or recharged) during a refreshoperation. A typical refresh operation comprises a sensing step fordetecting the logic state (e.g., ‘0’ or ‘1’) of stored data, and arefresh step for applying additional charge to the cells in accordancewith the detected logic state.

Unfortunately, different volatile memory cells tend to have differentcharge retention characteristics due to variances in memory fabricationprocesses. As a result, some memory cells may need to be recharged morefrequently than others. In other words, due to the different chargeretention characteristics, some memory cells lose data more quickly thanother memory cells and require the application of a refresh operationhaving a shorter period.

The presence of memory cells with different refresh requirements createsa number of potential problems for the overall design and timing ofvolatile memory devices. For instance, if all memory cells are requiredto be refreshed with the same period, the period must be short enough sothat data is not lost from the memory cells having the poorest chargeretention characteristics. This, however, results in inefficient powerconsumption. On the other hand, if different refresh timing is used fordifferent memory cells, the memory device may require complex additionalcircuitry and logic, resulting in inefficient use of chip area.

In sum, there is a tradeoff between the specificity of refresh timingand the complexity of refresh circuitry. As refresh timing becomes morespecific to the requirements of individual memory cells, the complexityof refresh circuitry tends to increase, and vice versa.

SUMMARY

Recognizing a need to account for the different charge retentioncharacteristics of different memory cells while limiting the complexityof refresh circuitry in a memory device, the inventive concept providesmethods, systems and devices in which multiple memory blocks arerefreshed simultaneously in groups, taking into account whether themultiple memory blocks include substandard memory cells requiringfrequent refreshing. Groups of memory blocks that include substandardmemory cells are refreshed with a first refresh period, and groups ofmemory blocks that do not include substandard memory cells are refreshedwith a second refresh period longer than the first refresh period. Thegroups of memory cells are generally organized in a manner that limitsthe number of memory blocks requiring frequent refreshing while alsolimiting the complexity of refresh circuitry.

According to one embodiment of the inventive concept, a method ofoperating a memory device is provided. The method identifies a firstplurality of memory blocks each comprising at least one substandardmemory cell, and further identifies a second plurality of memory blockseach comprising no substandard memory cells. The method generates a rowaddress code with a first value corresponding to two or more memoryblocks, where at least one of the two or more memory blocks is from thefirst plurality of memory blocks, and further generates a row addresscode with a second value corresponding to at least two memory blocksfrom the second plurality of memory blocks. Thereafter, the methodperforms a first refresh operation on the memory blocks corresponding tothe first row address code value using a first refresh period, andperforms a second refresh operation on the memory blocks correspondingto the second row address code value with a second refresh period longerthan the first refresh period.

According to another embodiment of the inventive concept, another methodof operating a memory device is provided. The method comprisesidentifying a row address code corresponding to a plurality of memoryblocks, wherein different values of the row address code identifydifferent ones of the plurality of memory blocks, determining whethereach of the plurality of memory blocks comprises at least onesubstandard memory cell, generating an abbreviated row address code byomitting one or more bits of the row address code based on which of thememory blocks comprises at least one substandard memory cell, andperforming a refresh operation by simultaneously refreshing memoryblocks having the same value of the abbreviated row address code.

According to still another embodiment of the inventive concept, yetanother method of performing a refresh operation is provided. The methodcomprises refreshing a first group of memory blocks using a firstrefresh period, and refreshing a second group of memory blocks using asecond refresh period longer than the first refresh period. The firstgroup comprises memory blocks with substandard memory cells and thesecond group comprises no memory blocks with substandard memory cells,and the grouping of the memory blocks into the first and second groupsminimizes the total number of memory blocks using the first refreshperiod while allowing memory cells with the same value of an abbreviatedrow address code to be refreshed simultaneously.

According to another embodiment of the inventive concept, a memorydevice is provided. the memory device comprises a memory cell arraycomprising a plurality of memory blocks each corresponding to adifferent value of a row address code, wherein a subset of the memoryblocks each comprise at least one substandard memory cell, and acontroller configured to perform a refresh operation on the plurality ofmemory blocks using an abbreviated row address code formed of a subsetof row address bits from the row address code, wherein each value of theabbreviated row address code corresponds to a group of at least two ofthe plurality of memory blocks. The memory device further comprises aselection circuit configured to select the abbreviated row address codefrom among a plurality of candidate abbreviated row address codes so asto minimize the number of groups having a memory block with at least onesubstandard memory cell.

According to yet another embodiment of the inventive concept, a memorysystem is provided. The memory system comprises a memory cell arraycomprising a plurality of memory blocks each corresponding to adifferent value of a row address code, wherein a subset of the memoryblocks each comprise at least one substandard memory cell. The systemfurther comprises a substandard memory cell detector configured todetect memory blocks having substandard memory cells, a controllerconfigured to perform a refresh operation on the plurality of memoryblocks using an abbreviated row address code formed of a subset of rowaddress bits from the row address code, wherein each value of theabbreviated row address code corresponds to a group of at least two ofthe plurality of memory blocks, and a selection circuit configured toselect the abbreviated row address code from among a plurality ofcandidate abbreviated row address codes so as to minimize the number ofgroups having a memory block with at least one substandard memory cell.

According to still another embodiment of the inventive concept, a memorydevice is provided. The memory device comprises a first plurality ofmemory blocks each comprising at least one substandard memory cell, asecond plurality of memory blocks each comprising no substandard memorycells, and a control circuit configured to generate a row address codewith a first value corresponding to two or more memory blocks, where atleast one of the two or more memory blocks is from the first pluralityof memory blocks, and further configured to generate a row address codewith a second value corresponding to at least two memory blocks from thesecond plurality of memory blocks. The memory device further comprises arefresh circuit configured to perform a first refresh operation on thememory blocks corresponding to the first row address code value using afirst refresh period, and further configured to perform a second refreshoperation on the memory blocks corresponding to the second row addresscode value with a second refresh period longer than the first refreshperiod.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the inventive concept will be more clearly understoodfrom the following detailed description taken in conjunction with theaccompanying drawings. In the drawings, like reference numbers denotelike features.

FIG. 1 is a block diagram illustrating a system comprising a memorydevice in accordance with an embodiment of the inventive concept.

FIG. 2 is a block diagram illustrating an example of the memory deviceshown in FIG. 1.

FIG. 3 is a block diagram illustrating an alternative configuration ofthe memory device illustrated in FIG. 2.

FIG. 4 is a block diagram illustrating a memory device and correspondingrow address codes in accordance with an embodiment of the inventiveconcept.

FIG. 5 is a block diagram illustrating the locations of substandardmemory cells within memory blocks of the memory device illustrated inFIG. 4.

FIG. 6 is a table illustrating row address code values corresponding tothe memory blocks of FIG. 5.

FIGS. 7A through 7D are tables illustrating abbreviated row addresscodes and corresponding memory blocks.

FIG. 8 is a flowchart illustrating a method of operating a memory deviceto refresh stored data in accordance with an embodiment of the inventiveconcept.

FIG. 9 is a block diagram illustrating a selector unit for selectingabbreviated row address codes to be used for refresh operations of amemory device in accordance with an embodiment of the inventive concept.

FIG. 10 is a block diagram illustrating a control circuit forcontrolling the timing of refresh operations in accordance with anembodiment of the inventive concept.

FIG. 11 is a waveform timing diagram illustrating signals used toperform a refresh operation in accordance with an embodiment of theinventive concept.

FIG. 12 is a block diagram illustrating a memory block selection signalgenerator in accordance with an embodiment of the inventive concept.

FIG. 13 is a block diagram illustrating a refresh period signalgenerator in accordance with an embodiment of the inventive concept.

FIG. 14A is a block diagram illustrating a refresh cycle controller inaccordance with an embodiment of the inventive concept.

FIG. 14B is a circuit diagram illustrating a fuse unit within therefresh cycle controller of FIG. 14A.

FIG. 14C is a table illustrating a relationship between different statesof the fuse unit of FIG. 14B and the operation of the refresh cyclecontroller.

FIG. 14D is a table further illustrating a relationship betweendifferent states of the fuse unit of FIG. 14B and the operation of therefresh cycle controller.

FIG. 15A is a circuit diagram illustrating an alternative fuse unitwithin the refresh cycle controller of FIG. 14A.

FIG. 15B is a table illustrating a relationship between different statesof the fuse unit of FIG. 15A and the operation of the refresh cyclecontroller.

FIG. 16 is a block diagram illustrating another alternativeconfiguration of the memory device illustrated in FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENTS

Selected embodiments of the inventive concept are described below withreference to the corresponding drawings. These embodiments are presentedas teaching examples and should not be interpreted to limit the scope ofthe inventive concept.

In general, embodiments of the inventive concept relate to memorydevices, systems, and methods designed to reduce the amount of powerrequired to execute refresh operations. For instance, some embodimentsreduce the power consumption of refresh operations by identifying memoryblocks containing memory cells with poor charge retentioncharacteristics, and refreshing those memory blocks as a group whilerefreshing other memory blocks separately. Groups that do not containany memory cells with poor charge retention characteristics can berefreshed less frequently than the groups that do contain such memorycells, reducing the overall power required to refresh all of the memoryblocks.

Memory cells that require a refresh operation more frequently thanothers because they fall below a predetermined quality standard aretermed “substandard” cells. The quality standard may be defined, forinstance, by how long the memory cells retain stored data in the absenceof power. As an example, memory cells that retain stored informationlonger than a predetermined interval (e.g., 256 ms) may be designated asnormal memory cells, while memory cells that do not retain storedinformation longer than the predetermined interval may be designated assubstandard memory cells.

To determine whether memory cells are normal or substandard, a detectionprocedure may be performed on the memory cells following manufacture.Such a detection procedure may involve, for instance, programming all ofthe memory cells to a particular value and then measuring their valuesas time passes. Cells that maintain their initial values after thepredetermined interval may be designated as normal, while those that donot may be designated as substandard.

In some embodiments, the process of detecting substandard memory cellsmay be performed using one or more components external to the memorydevice. For instance, the process may be performed by reading storeddata into a cache and using a central processing unit (CPU) to evaluateand record the locations of the substandard memory cells. In otherembodiments, the process could be performed by elements forming part ofthe memory device.

Once memory cells have been designated as normal or substandard, amemory chip or other device including the memory cells may be configuredso that memory blocks including the substandard memory cells arerefreshed in separate groups from memory blocks not includingsub-standard cells. Such configuration may be performed, for instance,by cutting selected fuses to alter refresh circuitry based on thelocations of the substandard memory cells, or by employing software orvarious alternative forms of reconfigurable hardware.

FIG. 1 shows a basic example of a system 10 in which selectedembodiments of the inventive concept may be implemented. System 10comprises a CPU 20, a user interface 30, a power supply 50, a memorydevice 40, and a memory controller 60, all connected to each other by abus and/or network architecture 70.

System 10 may take any of several different forms, such as a personalcomputer, workstation, networking device, gaming-console, handhelddevice, or embedded system, to name but a few. Additionally, system 10and its components may be distributed or divided into manysub-components, such as multiple CPU cores, distributed memories, adistributed bus or network architecture, remote user interfaces, and soon.

System 10 may be implemented as a portable or non-portable device.Accordingly, power supply 50 may comprise a portable source of power,such as a battery, or a stationary source of power, such as analternating current (AC) outlet. Moreover, power supply 50 may comprisemore than one source of power for powering different elements of system10.

Memory device 40 typically comprises at least one volatile memory suchas a DRAM. Moreover, memory device 40 may comprise more than one memorychip, which may include multiple types and sizes of memories, as well asmemories using different protocols. For instance, memory device 40 maycomprise a volatile memory such as a DRAM in combination with otherforms of memory such as a nonvolatile memory.

Memory device 40 and any constituent chips may be organized in any ofseveral different architectures. For instance, memory device 40 may beorganized in any number of chips, banks, memory blocks, sectors, and soon. Such elements may be arranged in hierarchies (e.g., L2, L3 caches,etc.), in parallel, or in a combination of hierarchical, parallel, orother configuration.

Memory device 40 may be packaged or integrated with other devices toform any of several different products or components. For instance,memory device 40 may be packaged in a memory card or other standalonememory product, or it may form part of a system-on-a-chip havingnumerous additional elements.

FIG. 2 illustrates an example configuration of memory device 40 inaccordance with one embodiment of the inventive concept. In thisexample, memory device 40 comprises a memory cell array 100, an addressdecoder 200, an input/output (IO) circuit 300, a control circuit 400, anaddress register 500, and a timing register 550.

Memory cell array 100 comprises a plurality of memory cells arranged inrows and columns, where each row is connected to a corresponding wordline and each column is connected to a corresponding bit line. Incertain embodiments, the memory cells comprise DRAM cells. The memorycells are typically divided into memory blocks, where each memory blockis refreshed as a unit, and all of the memory blocks are refreshed insuccession during a refresh cycle of memory cell array.

The memory blocks of memory cell array 100 are typically designated by aportion of a row address. For instance, assuming memory cell array 100comprises 2^(m) rows of memory cells addressable by an m-bit rowaddress, a sub-portion of the m-bit row address may designate rowsbelonging to the same memory block. As a more concrete example, assumememory cells of memory cell array 100 are addressable by a 13-bit rowaddress designated RA[12:0], where RA12 denotes a first bit, RA11denotes a second bit, etc.; the bits RA[12:9] may be used to designatedifferent memory blocks. In other words, memory cells having the samerow address bits RA[12:9] form part of the same memory block, andtherefore the bits RA[12:9] may be used as a row address (RA) code forselecting a memory block for different operations such as a refreshoperation. Because row address code RA[12:9] has four bits, it can beused to divide memory cell array 100 into sixteen memory blockscorresponding to the sixteen possible values of RA[12:9]: “0000,”“0001,” “0010”, . . . , “1111.”

Address decoder 200 comprises a row address decoder 210 and a columnaddress decoder 220. Address decoder 200 receives an address ADDR fromcontrol circuit 400 indicating a location of memory cells to beprogrammed in memory cell array 100. Address decoder 200 furtherreceives refresh cycle information indicating an order in which torefresh memory blocks during a refresh operation, and the duration of arefresh operation to be performed on each memory block. As an example,the refresh cycle information may indicate a refresh period tREF orprovide a refresh enable signal REF_EN for each refresh operation. Itmay also indicate that certain memory blocks will be refreshed at thesame time as each other.

For read and write operations, row address decoder 210 and columnaddress decoder 220 decode address ADDR to generate a row addressRA[12:0] and a column address CA[12:0] of selected memory cells. Forrefresh operations, row address decoder 210 and column address decoder220 use the refresh cycle information to control the order and durationof refresh signals applied to the memory blocks of memory cell array100.

IO circuit 300 comprises a data buffer for receiving and storing inputdata DATA to be written to memory cell array 100 in a write operation,and output data DATA to be read from memory cell array 100 in a readoperation.

Control circuit 400 comprises various elements for determining thetiming for refresh operations to be performed on memory cell array 100.Control circuit 400 receives address ADDR, a row address code selectionsignal S_RA, and an indication of the number of memory blocks in memorycell array 100. Based on this information, control circuit 400 generatesa refresh enable signal REF_EN to control refresh periods of differentmemory blocks and/or groups of memory blocks.

Address register 500 comprises storage elements and data for receiving alogical address ADDL and converting logical address ADDL into addressADDR. In other words, address register 500 implements a mapping schemefor mapping between a logical address and a physical address of memorycell array 100. Address register 500 receives logical address ADDL and aclock signal CLK from an external source and outputs address ADDR tocontrol signal 400.

Timing register 550 comprises timing elements for controlling theoperation of other elements of memory device 40. In particular, timingregister 550 comprises registers for storing and outputting clock signalCLK, a chip select signal CSB, a row address strobe signal RASB, acolumn address strobe signal CASB, a write enable signal WEB, and aclock enable signal CKE. Timing register 550 outputs these varioussignals to address decoder 200, control circuit 400, address register500, and IO data circuit 300 to control the timing of various operationsincluding the transfer of information to other elements.

FIG. 3 is a block diagram illustrating an alternative configuration ofmemory cell array 100 and row and column decoders 210 and 220. In thisconfiguration, memory cell array 100 is divided into a plurality ofbanks 100A through 100X, row decoder 210 is divided into a plurality ofrow decoders 210A through 210X, and column decoder 220 is divided into aplurality of column decoders 220A through 220X. Column decoders 220Athrough 220X each receive a column address CA[12:0] derived from addressADDR. Similarly, row decoders 210A through 210X receive a row addressRA[12:0] derived from address ADDR. Each of memory banks 100A through100X comprises a plurality of memory blocks that can be refreshed ingroups based on the locations of substandard memory cells, similar tothe configuration of FIG. 2.

FIG. 4 is a block diagram illustrating memory cell array 100 andcorresponding row address code values in accordance with an embodimentof the inventive concept. In this embodiment, memory cell array 100 isdivided into sixteen memory blocks 111, 112, 121, 122, 131, 132, 141,142, 151, 152, 161, 162, 171, 172, 181, and 182. Each of these memoryblocks corresponds to several rows of memory cells having row addressesRA[12:0]. More specifically, each memory block corresponds to a set ofmemory cells having the same row address bits RA[12:9].

For instance, as illustrated in FIG. 4, memory block 111 comprisesmemory cells having row address bits RA[12:9]=“0000”, memory block 112comprises memory cells having row address bits RA[12:9]=“0001”, and soon. During refresh operations, rows of memory cells having the samevalue of row address bits RA[12:9] (i.e., row address code RA[12:9]) arerefreshed at the same time. Moreover, as described in further detailbelow, rows of memory cells having a common abbreviated row address codemay be refreshed at the same time to reduce the complexity of requiredrefresh circuitry while limiting the overall frequency with which thememory blocks are refreshed.

FIG. 5 is a block diagram illustrating the locations of substandardmemory cells within memory blocks of memory cell array 100 illustratedin FIG. 4. The location of each substandard memory cell is illustratedby the letter “X”. Accordingly, substandard memory cells can beidentified in memory blocks 111, 121, 132, 151, 161, 171, and 181. Thus,memory cell array 100 comprises seven memory blocks with at least onesubstandard memory cell and nine memory blocks with no substandardmemory cell.

Because each of memory blocks 111, 121, 132, 151, 161, 171, and 181comprises at least one substandard memory cell, each of these memoryblocks must be refreshed more frequently (i.e., with a shorter refreshperiod) than the other memory blocks in order to ensure that thesubstandard memory cells do not lose stored data. Thus, if all of thememory blocks in memory cell array 100 were refreshed using the shorterrefresh cycle, the resulting power consumption would be inefficientbecause the nine memory blocks with no substandard memory cells requireless frequent refreshing. On the other hand, if all of the memory blocksin memory cell array 100 were refreshed individually to ensure morefrequent refreshing of each memory block with a substandard memory cell,and less frequent refreshing of each memory block without a substandardmemory cell, the resulting circuit configuration would be relativelycomplex. Accordingly, in selected embodiments of the inventive conceptdescribed below, a compromise is made so that memory blocks arerefreshed in groups to limit the required circuitry, and the groups arechosen so that memory blocks having substandard memory cells arerefreshed simultaneously with other memory blocks having substandardmemory cells in order to limit inefficient power consumption.

FIG. 6 is a table illustrating row address code values corresponding tothe memory blocks of FIG. 5. In particular, each row of the tableillustrates the row address bits RA12, RA11, RA10 and RA09 associatedwith a particular one of memory blocks 111 through 182. Arrows on theleft side of FIG. 6 indicate memory blocks that include at least onesubstandard memory cell.

FIGS. 7A through 7D are tables illustrating abbreviated row addresscodes and values derived from the row address code RA[12:9] andcorresponding values illustrated in FIG. 6. In particular, FIG. 7Aillustrates an abbreviated row address code RA[12,11,10], which isgenerated by omitting row address bit RA09 from row address codeRA[12:9]. Similarly, FIG. 7B illustrates an abbreviated row address codeRA[12,11,09], which is generated by omitting row address bit RA10 fromrow address code RA[12:9]. FIG. 7C illustrates an abbreviated rowaddress code RA[12,10,09], which is generated by omitting row addressbit RA11 from row address code RA[12:9]. FIG. 7D illustrates anabbreviated row address code RA[11,10,09], which is generated byomitting row address bit RA12 from row address code RA[12:9]. In each ofFIGS. 7A through 7D, arrows on the left side indicate memory blocks thatcontain at least one substandard memory cell.

The abbreviated row address codes illustrated in FIGS. 7A through 7D arecandidate codes that may potentially be used to group together differentmemory blocks for refresh operations. Each candidate abbreviated rowaddress code defines a distinct grouping of memory blocks into groupslabeled “a,” through “h,” where the members of each group have the samevalue of the abbreviated row address code. For instance, in FIG. 7A,group “a” comprises memory blocks 111 and 112, which have the same value“000” of abbreviated row address code RA[12,11,10]. Similarly, group “b”in FIG. 7A comprises memory blocks 121 and 122 having the same value“001” of abbreviated row address code RA[12,11,10]. The candidate codethat produces the minimum number of groups having at least onesubstandard memory cell will be used as the grouping for refreshoperations.

In FIG. 7A, group “a” comprises memory blocks 111 and 112, whichcorrespond to abbreviated row address code RA[12,11,10] value “000”.Group “b” comprises memory blocks 121 and 122, which correspond toabbreviated row address code RA[12,11,10] value “001.” Group “c”comprises memory blocks 131 and 132, which correspond to abbreviated rowaddress code RA[12,11,10] value “010.” Group “d” comprises memory blocks141 and 142, which correspond to abbreviated row address codeRA[12,11,10] value “011.” Group “e” comprises memory blocks 151 and 152,which correspond to abbreviated row address code RA[12,11,10] value“100.” Group “f” comprises memory blocks 161 and 162, which correspondto abbreviated row address code RA[12,11,10] value “101.” Group “g”comprises memory blocks 171 and 172, which correspond to abbreviated rowaddress code RA[12,11,10] value “110.” Group “h” comprises memory blocks181 and 182, which correspond to abbreviated row address codeRA[12,11,10] value “111.”

In the grouping of FIG. 7A, seven out of the eight groups include atleast one substandard memory cell. In particular, there is at least onesubstandard memory cell in groups “a,” “b,” “c,” “e,” “f,” “g,” and “h”.Thus, if the memory blocks in each group are refreshed together, sevenout of the eight groups will require a relatively shorter refresh perioddue to the presence of the substandard memory cells. As a result, thegroupings of FIG. 7A result in only minimal reduction of the powerconsumption compared with a scenario where all of the memory blocks arerefreshed with the relatively shorter refresh period.

In FIG. 7B, group “a” comprises memory blocks 111 and 121, whichcorrespond to abbreviated row address code RA[12,11,09] value “000”.Group “b” comprises memory blocks 112 and 122, which correspond toabbreviated row address code RA[12,11,09] value “001.” Group “c”comprises memory blocks 131 and 141, which correspond to abbreviated rowaddress code RA[12,11,09] value “010.” Group “d” comprises memory blocks132 and 142, which correspond to abbreviated row address codeRA[12,11,09] value “011.” Group “e” comprises memory blocks 151 and 161,which correspond to abbreviated row address code RA[12,11,09] value“100.” Group “f” comprises memory blocks 152 and 162, which correspondto abbreviated row address code RA[12,11,09] value “101.” Group “g”comprises memory blocks 171 and 181, which correspond to abbreviated rowaddress code RA[12,11,09] value “110.” Group “h” comprises memory blocks172 and 182, which correspond to abbreviated row address codeRA[12,11,09] value “111.”

In the grouping of FIG. 7B, only four out of the eight groups include atleast one substandard memory cell. In particular, the only groups withat least one substandard memory cell are groups “a,” “d,” “e,” and “g”.Thus, if the memory blocks in each group are refreshed together, onlyfour out of the eight groups will require a relatively shorter refreshperiod due to the presence of the substandard memory cells. As a result,the groupings of FIG. 7B produce more efficient power consumptioncompared with the grouping of FIG. 7A. As a result, the abbreviated rowaddress code RA[12,11,09] used to produce the grouping of FIG. 7B isconsidered to be a better candidate code than the abbreviated rowaddress code RA[12,11,10] used to produce the groupings of FIG. 7A.

In FIG. 7C, group “a” comprises memory blocks 111 and 131, whichcorrespond to abbreviated row address code RA[12,10,09] value “000”.Group “b” comprises memory blocks 112 and 132, which correspond toabbreviated row address code RA[12,10,09] value “001.” Group “c”comprises memory blocks 121 and 141, which correspond to abbreviated rowaddress code RA[12,10,09] value “010.” Group “d” comprises memory blocks121 and 142, which correspond to abbreviated row address codeRA[12,10,09] value “011.” Group “e” comprises memory blocks 151 and 171,which correspond to abbreviated row address code RA[12,10,09] value“100.” Group “f” comprises memory blocks 152 and 172, which correspondto abbreviated row address code RA[12,10,09] value “101.” Group “g”comprises memory blocks 161 and 181, which correspond to abbreviated rowaddress code RA[12,10,09] value “110.” Group “h” comprises memory blocks162 and 182, which correspond to abbreviated row address codeRA[12,10,09] value “111.”

In the grouping of FIG. 7C, five out of the eight groups include atleast one substandard memory cell. In particular, there is at least onesubstandard memory cell in groups “a,” “b,” “c,” “e” and “g”. Thus, ifthe memory blocks in each group are refreshed together, five out of theeight groups will require a relatively shorter refresh period due to thepresence of the substandard memory cells. As a result, the grouping ofFIG. 7C results in more efficient power consumption than the grouping ofFIG. 7A, but less efficient power consumption than the grouping of FIG.7B.

In FIG. 7D, group “a” comprises memory blocks 111 and 151, whichcorrespond to abbreviated row address code RA[11,10,09] value “000”.Group “b” comprises memory blocks 112 and 132, which correspond toabbreviated row address code RA[11,10,09] value “001.” Group “c”comprises memory blocks 121 and 141, which correspond to abbreviated rowaddress code RA[11,10,09] value “010.” Group “d” comprises memory blocks121 and 142, which correspond to abbreviated row address codeRA[11,10,09] value “011.” Group “e” comprises memory blocks 151 and 171,which correspond to abbreviated row address code RA[11,10,09] value“100.” Group “f” comprises memory blocks 152 and 172, which correspondto abbreviated row address code RA[11,10,09] value “101.” Group “g”comprises memory blocks 161 and 181, which correspond to abbreviated rowaddress code RA[11,10,09] value “110.” Group “h” comprises memory blocks162 and 182, which correspond to abbreviated row address codeRA[11,10,09] value “111.”

In the grouping of FIG. 7D, five out of the eight groups include atleast one substandard memory cell. In particular, there is at least onesubstandard memory cell in groups “a,” “c,” “f,” “e” and “g”. Thus, ifthe memory blocks in each group are refreshed together, five out of theeight groups will require a relatively shorter refresh period due to thepresence of the substandard memory cells. As a result, the grouping ofFIG. 7D results in more efficient power consumption than the grouping ofFIG. 7A, but less efficient power consumption than the grouping of FIG.7B.

Because the grouping of FIG. 7B results in fewer groups having at leastone substandard memory cell, the abbreviated row address code of FIG. 7Bwill be used to perform refresh operations instead of the othercandidate abbreviated row address codes in FIGS. 7A, 7C, and 7D.

FIG. 8 is a flowchart illustrating a method 800 for operating a memorydevice to refresh stored data in accordance with an embodiment of theinventive concept. In the description that follows, example method stepsare indicated by parentheses (xxx).

In the method of FIG. 8, a memory device is tested to determine thelocations of substandard memory cells (805). Such a test may comprise,for instance, programming memory cells of multiple memory blocks to apredetermined logic state and then detecting the ability of eachprogrammed memory cell to retain the predetermined logic state overtime. The detection of each memory cell's ability to retain thepredetermined logic state may be performed, for instance, by readingstored information from the memory device following a brief delay afterthe memory cells are programmed, such as 256 ns. If the storedinformation read from a memory cell after the brief delay does notcorrespond to the programmed logic state, the memory cell may be deemeda substandard memory cell. Otherwise, the memory cell may be deemed anormal memory cell.

The analysis of stored information to determine the locations ofsubstandard memory cells may be performed by a device either internal orexternal to the memory device. As an example, an external device such asa CPU could read data out of the memory blocks and determine whether anyof the data indicates a substandard memory cell.

After the memory device has been tested to determine the locations ofsubstandard memory cells, an abbreviated row address code is thengenerated to create groupings of memory blocks that limit the number ofmemory blocks requiring frequent refreshing (810). In one embodiment,the abbreviated row address code is generated by considering a number ofcandidate codes each omitting a different row address bit from a rowaddress code, and then identifying the candidate with the fewest numberof groups having at least one substandard memory cell as the abbreviatedrow address code to be used for refresh operations.

Once the abbreviated row address code has been identified, a refreshoperation is performed simultaneously on memory blocks having the samevalue of the abbreviated row address code (815). Moreover, using theabbreviated row address code to cycle through memory blocks, a refreshoperation is performed sequentially on groups of memory cells such asthose illustrated in FIG. 7 to complete a refresh cycle.

FIG. 9 is a block diagram illustrating a selector unit 900 for selectingrow address codes to be used for refresh operations of a memory devicein accordance with an embodiment of the inventive concept. Selector unit900 comprises a plurality of RA selectors 905 each receiving an enablesignal corresponding to a bit of row address RA[12:0] and selectivelyoutputting the corresponding bit based on an abbreviated row addresscode to be used for refresh operations. For instance, where theabbreviated row address code RA[12,11,09], illustrated in FIG. 7B, isused to perform refresh operations, selector 900 receives thecorresponding enable signals RA12_EN, RA11_EN, and RA09_EN and generatesrow address code selection signal S_RA based on the values of rowaddress bits RA12, RA11, and RA09.

FIG. 10 is a block diagram illustrating an embodiment of control circuit400. In this embodiment, control circuit 400 comprises a counter 410, amemory block selection signal generator 420, a refresh cycle controller430, a refresh period signal generator 440, and a reset circuit 450.

Counter 410 receives a number of memory block groups to be refreshedduring a refresh cycle. In the example of FIG. 7, for instance, thenumber of groups is eight, including groups “a”-“h”. Based on the numberof groups received, counter 410 generates a counter signal CNTS tocontrol the timing of refresh operations.

Memory block selection signal generator 420 receives counter signal CNTSand a reset signal RST and generates block selection signals BLK[1:8] insynchronization with counter signal CNTS. Block selection signalsBLK[1:8] are successively asserted during a refresh cycle so thatrefresh operations are performed in succession on memory block groups“a”-“h”, which correspond to indices 1 through 8. Reset signal RSTresets index memory block selection signal generator 420 and refreshperiod signal generator 440 to initial values when control circuit 400is initialized. Block selection signal BLK1 and refresh period signalPRD1 are asserted before other signals following initialization ofelements 420 and 440.

Refresh cycle controller 430 receives block selection signals BLK[1:8],row address code selection signal S_RA, and a refresh period signalsPRD[1:4], and generates refresh enable signal REF_EN based on thereceived signals. Block selection signals BLK[1:8] indicate a group ofmemory blocks to be refreshed at the current stage of a refresh cycle,and refresh period signals PRD[1:4] indicate a current refresh period.Row address code S_RA indicates which row address bits define thegrouping of memory blocks and therefore the order in which the memoryblocks are to be refreshed. Together, block selection signals BLK[1:8],refresh period signals PRD[1:4], and row address code selection signalS_RA determine which memory blocks are to be refreshed during aparticular time interval. Accordingly, the value of refresh enablesignal REF_EN is determined by the combination of these signals.

Refresh cycle controller 430 may further store or receive informationindicating which groups of memory blocks use a relatively shorterrefresh period and which groups use a relatively longer refresh period.Memory blocks using a relatively shorter refresh period may berefreshed, for instance, during each of refresh periods PRD1 throughPRD4, while memory blocks using a relatively longer refresh period maybe refreshed during only one or two of refresh periods PRD1 throughPRD4.

To illustrate the operation of refresh cycle controller 430, suppose rowaddress code selection signal indicates that abbreviated row addresscode RA[12,11,09] of FIG. 7B designates groups of memory blocks to berefreshed together and block selection signals BLK[1:8] and refreshperiod signals PRD[1:4] indicate which of the groups may be refreshedcurrently. In this case, since group “a” includes substandard memorycells, group “a” may be refreshed each time block selection signal BLK1is asserted during a refresh cycle. On the other hand, since group “b”does not include any substandard memory cells, group “b” may berefreshed only when both block selection signal BLK2 is asserted andrefresh period signal PRD1 is asserted. Where group “a” is refreshedregardless of the value of refresh period signals PRD[1:4] and group “b”is refreshed only when refresh period PRD1 is asserted, the memoryblocks of group “b” will be refreshed four times less frequently thanthe memory blocks of group “a”.

Refresh period signal generator 440 receives block selection signalsBLK[1:8] and reset signal RST and generates refresh period signalsPRD[1:4] in synchronization with block selection signals BLK[1:8].Refresh period signals PRD[1:4] are asserted in succession, with eachsuccessive signal remaining asserted for a complete cycle of blockselection signals BLK[1:8].

Reset circuit 450 generates reset signal RST to reset memory blockselection signal generator 420 and refresh period signal generator 440.Such reset operations may occur, for instance, when control circuit 400is initialized, or in response to another event.

FIG. 11 is a waveform timing diagram illustrating the operation ofcontrol circuit 400 in accordance with one embodiment of the inventiveconcept. In the example of FIG. 11, control circuit 400 performs refreshoperations in four periods P1, P2, P3, and P4, which correspond torefresh period signals PRD1, PRD2, PRD3, and PRD4, respectively.

During each period, counter signal CNTS passes through eight cycles,during which block selection signals BLK[1:8] are successively asserted.Meanwhile, refresh period signals PRD1, PRD2, PRD3, and PRD4 areasserted in succession during periods P1, P2, P3, and P4. In order toprovide a longer refresh period to some groups of memory blocks whileproviding a shorter refresh period to other groups of memory blocks,some groups of memory blocks are refreshed during each of periods P1-P4,while others are refreshed only during a subset of periods P1-P4. Inthis way, refresh period signals PRD[1:4] can be used to control therefresh rate of different groups of memory blocks.

FIG. 12 is a block diagram illustrating an embodiment of memory blockselection signal generator 420. In this embodiment, memory blockselection signal generator 420 comprises a shift register comprisingeight data flip flops DFF 421 through DFF 428. These flip flops transfera single bit of data in synchronization with counter signal CNTS togenerate the pattern of block selection signals BLK[1:8] illustrated inFIG. 11.

FIG. 13 is a block diagram illustrating an embodiment of refresh periodsignal generator 440. In this embodiment, refresh period signalgenerator 440 comprises a shift register comprising four data flip flopsDFF 41 through DFF 444. These flip flops transfer a single bit of datain synchronization with block selection signal BLK1 to generate thepattern of refresh period signals PRD[1:4] illustrated in FIG. 11.

FIG. 14A is a block diagram illustrating an embodiment of refresh cyclecontroller 430. In this embodiment, refresh cycle controller 430comprises a fuse unit 460 and refresh enable signal generator 480.

Fuse unit 460 receives one of block selection signals BLK[1:8], whichwill be referred to as block selection signal BLKi, and generates acorresponding fuse signal FUSEi. The value of fuse signal FUSEi dependson whether block selection signal BLKi corresponds to a group of memoryblocks including at least one substandard memory cell. In particular,where block selection signal BLKi corresponds to a group of memoryblocks with at least one substandard memory cell, fuse signal FUSEiassumes a logic state that causes a refresh operation to be performedduring each of periods P1 through P4 in FIG. 11. Otherwise, whereselection signal BLKi corresponds to a group of memory blocks withoutany substandard memory cells, fuse signal FUSEi assumes a logic statethat causes a refresh operation to be performed during only a subset ofperiods P1 through P4. To establish the logical relationship betweenblock selection signal BLKi and fuse signal FUSEi, a fuse may be cutwithin fuse unit 460 upon determining the groups in which memory blocksare to be refreshed. The manner of cutting the fuses will be describedin further detail with reference to FIGS. 14B through 14D below.

Refresh enable signal generator 480 receives row address code selectionsignal S_RA, refresh period signals PRD[1:4], and fuse signal FUSEi, andgenerates refresh enable signal REF_EN to refresh group “i” based on thevalues of the received signals. For instance, using block selectionsignal BLK1, and row address code selection signal S_RA set to RA12,RA11, RA09 as in FIG. 7B, refresh enable signal generator 480 controlsrefresh enable signal REF_EN so that memory blocks 111 and 121 arerefreshed together during each of periods P1 through P4. In this case,the value of selection signal S_RA causes memory blocks 111 and 121 tobe refreshed together, the value of FUSEi causes these memory blocks tobe refreshed during each of periods P1 through P4, and the refreshperiod signals PRD[1:4] indicate the timing of the periods P1 through P4to control the timing so that refresh enable signal REF_EN is assertedduring appropriate intervals.

FIG. 14B is a circuit diagram illustrating a fuse circuit 461 containedin an embodiment of fuse unit 460. FIG. 14C is a table illustrating arelationship between different states of the fuse unit of FIG. 14B andthe operation of the refresh cycle controller. FIG. 14D is a tablefurther illustrating a relationship between different states of the fuseunit of FIG. 14B and the operation of the refresh cycle controller.

Fuse circuit 461 comprises a fuse 462, an inverter 463, a voltage sourceVDD, ground VSS, transistors M1 and M2, and an input terminal receivingblock selection signal BLKi. Where block selection signal BLKicorresponds to a group of memory blocks containing at least onesubstandard memory cell, fuse 462 is not cut. As a result, fuse signalFUSEi assumes a logic state “0”, causing refresh operations to beperformed during each of the periods P1 through P4 on the memory blockscorresponding to block selection signal BLKi, as indicated in the tableof FIG. 14C. On the other hand, where block selection signal BLKicorresponds to a group of memory blocks containing no substandard memorycells, fuse 462 is cut. As a result, fuse signal FUSEi assumes a logicstate “1”, causing refresh operations to be performed only duringperiods P1 and P3 on the memory blocks corresponding to block selectionsignal BLKi, as indicated in the table of FIG. 14C. In other words,refresh operations are performed twice as frequently when fuse signalFUSEi is in logic state “0” as opposed to logic state “1”.

As indicated by FIG. 14D, fuse 462 is not cut in fuse circuits 461corresponding to memory blocks with substandard memory cells, and fuse462 is cut in fuse circuits 461 corresponding to memory blocks with nosubstandard memory cells. It should be recognized, however, that therelationship in FIG. 14D between cut fuses and memory blocks withsubstandard memory cells is a design choice and could be reversed ormodified in various ways.

FIG. 15A is a circuit diagram illustrating an alternative fuse circuit465 within the refresh cycle controller of FIG. 14A. FIG. 15B is a tableillustrating a relationship between different states of the fuse unit ofFIG. 15A and the operation of the refresh cycle controller.

Fuse circuit 465 functions similar to fuse circuit 461 except that fusecircuit 465 can assume four different configurations allowing refreshoperations to be performed during different combinations of periods P1through P4. The four different configurations of fuse circuit 465 areobtained by cutting or not cutting fuses 466 and 468. Where fuses 466and 468 remain uncut, fuse signals FUSEi1 and FUSEi2 assume a logicstate “00,” resulting in a refresh operation during each of periods P1through P4 as indicated in FIG. 15B. Similarly, where fuses 466 and 468are cut or not cut in the remaining alternative combinations, fusesignals FUSEi1 and FUSEi2 assume logic states “01”, “10”, and “11” toproduce refresh operations during various periods shown in FIG. 15B. Byusing the different refresh patterns shown in FIG. 15B, different memoryblocks can be refreshed with more than two different intervals toaccommodate substandard cells with varying grades of charge retentioncharacteristics.

FIG. 16 is a block diagram illustrating another alternativeconfiguration of memory device 40 illustrated in FIG. 1. Thisconfiguration is similar to that illustrated in FIG. 2, except that theconfiguration of FIG. 16 comprises various additional components forcontrolling memory access operations and refresh operations, and has amodified organization of the elements in control circuit 400. Forinstance, the configuration of FIG. 16 comprises a refresh state (SREF)controller 540 for controlling a refresh state of memory device 40 andgenerating an oscillation signal OSC. Additionally, this configurationincorporates elements 420, 430, 440, and 450 of control circuit 400 intoa refresh control unit 510 and incorporates element 410 into a refreshcounter 530.

As illustrated by the foregoing, the inventive concept provides methods,devices, and systems in which memory blocks having substandard memorycells are refreshed together while memory blocks having no substandardmemory cells are refreshed together. As a result, these methods, systemsand devices tend to limit inefficient power consumption while alsolimiting inefficient use of chip area.

Finally, the foregoing is illustrative of selected embodiments and isnot to be construed as limiting thereof. Many modifications may be madeto the embodiments without materially departing from the novel teachingsand advantages of the inventive concept. Accordingly, all suchmodifications may be included within the scope of the inventive conceptas defined by the claims.

What is claimed is:
 1. A method of operating a memory device,comprising: identifying a first plurality of memory blocks eachcomprising at least one substandard memory cell; identifying a secondplurality of memory blocks each comprising no substandard memory cells;generating a plurality of candidate first row address codes by omittingdifferent one or more bits of a row address corresponding to two or morememory blocks, where at least one of the two or more memory blocks isfrom the first plurality of memory blocks; selecting one of theplurality of candidate first row address codes as a first row addresscode; generating a second row address code by omitting one or more bitsof a row address corresponding to at least two memory blocks from thesecond plurality of memory blocks; performing a first refresh operationsimultaneously on the memory blocks having the first row address codevalue using a first refresh period; and performing a second refreshoperation simultaneously on the memory blocks having the second rowaddress code value with a second refresh period longer than the firstrefresh period.
 2. The method of claim 1, wherein identifying the firstplurality of memory blocks comprises: programming memory cells ofmultiple memory blocks in the memory device to a predetermined logicstate; and detecting the ability of each programmed memory cell toretain the predetermined logic state over time.
 3. The method of claim2, further comprising: upon determining that one or more memory cellswithin a selected memory block fails to retain stored information forthe second refresh period, assigning the selected memory block to thefirst plurality.
 4. The method of claim 1, further comprising: uponidentifying the first plurality of memory blocks each comprising atleast one substandard memory cell, cutting selected fuses of a controlcircuit to adjust the timing of refresh operations to be performed onthe first plurality of memory blocks to the first refresh period.
 5. Amethod of operating a memory device, comprising: identifying a firstplurality of memory blocks each comprising at least one substandardmemory cell; identifying a second plurality of memory blocks eachcomprising no substandard memory cells; generating a first row addresscode by omitting one or more bits of a row address corresponding to twoor more memory blocks, where at least one of the two or more memoryblocks is from the first plurality of memory blocks; generating a secondrow address code by omitting one or more bits of a row addresscorresponding to at least two memory blocks from the second plurality ofmemory blocks; performing a first refresh operation simultaneously onthe memory blocks having the first row address code value using a firstrefresh period; and performing a second refresh operation simultaneouslyon the memory blocks having the second row address code value with asecond refresh period longer than the first refresh period, whereingenerating the first row address code comprises: identifying a pluralityof unabbreviated row address code values each having a bit-length “N”and corresponding to a single memory block; identifying a plurality ofabbreviated row address code values each having a bit-length less than“N” and corresponding to more than one of the unabbreviated row addresscode values and the corresponding memory blocks; and distinguishing oneof the abbreviated row address code values as the first value upondetermining that the one of the abbreviated row address code valuescorresponds to at least one memory block comprising a substandard memorycell.
 6. The method of claim 5, further comprising: determining amapping between the unabbreviated row address code values andabbreviated row address code values by omitting one or more bits of theunabbreviated row address code values based on the locations ofsubstandard memory cells within the memory blocks corresponding to theunabbreviated row address code values.
 7. The method of claim 6, whereinthe mapping minimizes the number of abbreviated row address code valuescorresponding to at least one memory block having one or moresubstandard memory cells.
 8. The method of claim 1, wherein the memorydevice comprises a dynamic random access memory.
 9. The method of claim1, wherein the second refresh period is at least 256 ns.
 10. A method ofoperating a memory device, comprising: identifying a row address codecorresponding to a plurality of memory blocks, wherein different valuesof the row address code identify different ones of the plurality ofmemory blocks; determining whether each of the plurality of memoryblocks comprises at least one substandard memory cell; generating aplurality of candidate abbreviated row address codes by omittingdifferent one or more bits of the row address code based on which of thememory blocks comprises at least one substandard memory cell; selectingone of the plurality of candidate abbreviated row address codes as anabbreviated row address code; and performing a refresh operation bysimultaneously refreshing memory blocks having the same value of theabbreviated row address code.
 11. The method of claim 10, furthercomprising: refreshing all of the memory blocks corresponding to aparticular value of the abbreviated row address code using a firstrefresh period if at least one of the memory blocks corresponding to theparticular value comprises a substandard memory cell; and refreshing allof the memory blocks corresponding to the particular value using asecond refresh period longer than the first period if none of the memoryblocks corresponding to the particular value comprises a substandardmemory cell.
 12. The method of claim 11, wherein the second refreshperiod is at least double the first refresh period.
 13. The method ofclaim 10, wherein said generating the plurality of candidate abbreviatedrow address codes comprises: generating a first candidate abbreviatedrow address code by omitting a first row address bit from the rowaddress code; and generating a second candidate abbreviated row addresscode by omitting a second row address bit from the row address codewhile retaining the first row address bit in the row address code, andsaid selecting one of the plurality of candidate abbreviated row addresscodes comprises: identifying a first set of groups of memory blocks,where each group comprises at least two memory blocks having a samevalue of the first candidate abbreviated row address code; identifying asecond set of groups of memory blocks, where each group comprises atleast two memory blocks having a same value of the second candidateabbreviated row address code; determining a first number indicating howmany groups within the first set have at least one memory block with asubstandard memory cell; determining a second number indicating how manygroups within the second set have at least one memory block with asubstandard cell; and assigning the first or second candidateabbreviated row address code as the abbreviated row address code basedon whether the first number is larger than the second number or viceversa.
 14. The method of claim 10, wherein the refresh operation isperformed with a first refresh period if one or more of thesimultaneously refreshed memory blocks has a substandard memory cell,and otherwise, the refresh operation is performed with a second refreshperiod longer than the first refresh period.
 15. A method of performinga refresh operation in a memory device, comprising: refreshing a firstgroup of memory blocks using a first refresh period; and refreshing asecond group of memory blocks using a second refresh period longer thanthe first refresh period; wherein the first group comprises memoryblocks with substandard memory cells and the second group comprises nomemory blocks with substandard memory cells; selecting an abbreviatedrow address code from a plurality of candidate abbreviated row addresscodes; and wherein the grouping of the memory blocks into the first andsecond groups minimizes the total number of memory blocks using thefirst refresh period while allowing memory cells with the same value ofthe abbreviated row address code to be refreshed simultaneously.
 16. Themethod of claim 15, further comprising: receiving an indication of thememory blocks having substandard memory cells, wherein the abbreviatedrow address code is selected based on which memory blocks have thesubstandard memory cells.
 17. The method of claim 15, wherein the memorydevice is a dynamic random access memory.
 18. The method of claim 15,wherein the second refresh period is at least double the first refreshperiod.
 19. The method of claim 15, further comprising: cutting selectedfuses of a control circuit to adjust the timing of refresh operations tobe performed on the first group of memory blocks to the first refreshperiod.
 20. The method of claim 15, further comprising: refreshing athird group of memory blocks using a third refresh period shorter thanthe first refresh period; wherein the third group comprises memoryblocks with substandard memory cells.